Investigating the role of microbial communities in the treatment performance of constructed wetlands: A comprehensive analysis of spatial variation, media and substrata

Abstract

Despite water scarcity and water pollution being critically escalating concerns worldwide, the discharge of untreated or poorly treated wastewater (WW) into the environment continues to be a constant practice in especially in developing countries in Sub-Saharan Africa. Among several other technologies, constructed wetlands have emerged as a reliable and cost-effective mechanism for the treatment of wastewater prior to discharge into the environment. However, our understanding of the resident microbial communities and their impact on Constructed Wetland (CW) performance remains surprisingly limited. This knowledge gap hinders the optimization of CW design and operation for improved wastewater treatment. Studying the microbial communities in CWs will thus enable an understanding of the biological processes taking place in CWs, which can be used to identify optimal operating conditions for enhanced performance. The main objective of this research was to assess the microbial communities involved in the bioremediation of wastewater in constructed wetlands. Specifically, this study assessed the effect of microbial communities attached to different media (gravel, plant roots, and water) on the treatment performance of CWs, investigated the spatial variation of bacterial communities within the constructed wetland, and examined the effect of substrata and macrophyte plants on the microbial community and treatment performance. This study employed a model treatment system comprising an Anaerobic Baffled Reactor (ABR) coupled with the CW located behind Africa Hall in Makerere University. Microbial communities on different media and from different locations were quantified and characterized by culture-dependent methods. Subsequently, the functional roles of these communities were determined by analyzing the removal efficiency of various WW parameters across different CW sections and correlating this data with the identified bacterial species and their abundance. The study revealed significant spatial variation in bacterial community composition throughout the constructed wetland. It revealed a general decrease in bacterial diversity downstream, suggesting a selective process for pollutant degradation. Notably, the highest bacterial populations were found associated with plant roots, highlighting the crucial role of macrophytes in enhancing CW treatment performance. Additionally, the presence of substrate media further enriched the bacterial community and improved treatment efficiency. The findings revealed that while phosphate removal was not significant, CWs with macrophytes and substrate media exhibited superior removal of biochemical oxygen demand (BOD). Interestingly, the presence of macrophytes slightly reduced total suspended solids (TSS) removal but significantly enhanced nitrate removal. Furthermore, the study suggests a potential antagonistic relationship between coliforms and lactobacillales, and identifies Pseudomonas as a key player in nitrate and BOD removal. These findings provide valuable insights into the variation of microbial communities and their role in the treatment performance of CWs, highlighting the importance of considering the selection of substrate materials and the presence of plants in these systems. By exploring the mechanisms of CW microbial communities, this research avails knowledge that can be used to optimize CW design and operation, ultimately enhancing CW effectiveness as a sustainable, eco-friendly and low-cost solution for wastewater treatment globally.